1. Field of the Invention
This invention relates to the formation of sintered polycrystalline beta alumina ceramic articles and to high density precursors thereof. More particularly, the invention relates to a practical method for forming such articles.
2. Prior Art
Molten sodium batteries, such as sodium-sulfur and sodium-antimony trichloride batteries, were developed more than a decade ago, and have promise for future widespread use in load leveling energy storage networks. Technical problems must be overcome, however, before commercial molten sodium batteries can be marketed. Among the most critical problems awaiting solution is the invention and development of technically adequate electrolyte membranes at an acceptable price.
A number of solid electrolyte phases have been tested for use an electrolytes and separators in molten sodium batteries. Among these are beta-alumina (U.S. Pat. No. 3,404,035 to Kummer et al) and beta"-alumina U.S. Pat. No. 3,475,225 to Tennenhouse et al). The term "beta alumina," as used herein, encompasses beta and beta" aluminas or mixtures thereof unless otherwise specified. Basis for the possible utility of beta alumina as the solid electrolyte for molten sodium batteries is the fact that beta aluminas are resistant to alkali attack and they are fast ion conductors in the interlayer plane directions. The solid electrolytes, used in the form of membranes, must possess high electronic resistivity along with low ionic resistivity. The ionic resistivity of mixtures of beta and beta" aluminas has been shown in decrease continuously as the fraction of beta" alumina increases. Consequently beta" alumina is generally the preferred phase.
Beta alumina membranes must also be rigid, dense and strong enough to contain the reactants during repeated electrochemical, mechanical and possibly thermal cycling in molten sodium and sodium polysulfide or antimony trichloride environments. In addition to meeting these technical requirements beta alumina electrolytes must be available at a price commensurate with an economical battery system. Therefore the membrane must be easily mass produced and possess a reasonably long shelf life.
Early in the development of molten sodium batteries certain but not all of these requirements were met. In accordance with the teachings of U.S. Pat. No. 3,404,035 (supra), crushed beta alumina refractory brick was employed to produce structures with desired high density (thus absence of open porosity). The structure also possessed desirably low sodium resistivities at 300.degree. C. of less than 30 ohm-cm. Shortly thereafter Tennenhouse et al (U.S. Pat. No. 3,475,225 supra) produced a lithium doped beta" alumina phase with a sodium resistivity at 300.degree. C. of less than 5 ohm-cm. Comparable low resistivity was achieved with a magnesium doped beta" alumina (U.S. Pat. No. 3,488,271 to Kummer et al and U.S. Pat. No. 3,719,531 to Dzieiuch et al).
Prior art methods for making polycrystalline beta alumina articles are generally characterized by the fact that initially a uniform mixture of sources of oxides of sodium and aluminum, and optionally oxides of lithium or magnesium, are uniformly mixed, using dry or wet milling. When wet mixing is employed the mixtures must be dried. The mixtures are then calcined to decompose the raw material and convert the reactants to beta alumina. The calcined beta alumina is then milled, using wet or dry milling, to place it in desired fine particle size form. To convert the beta alumina powder into formed articles useful as membranes for battery systems, the powder must undergo a series of steps including pressing and sintering. Generally one or more organic processing aids must be incorporated with the powdered beta alumina before any of these steps is carried out. In most cases a processing aid is incorporated with previously milled beta alumina powder by slurrying the powder with the organic processing aid in an organic vehicle which dissolves the processing aid and then removing the volatile vehicle. The resulting powder is then pressed, typically by isostatic pressing, into an article of desired shape, such as a thin-walled tube. Typically pressures.gtoreq.40 Kpsi must be used in order to produce finished articles of desired high density. The pressed shapes must then be heated to eliminate the organics and sinter the powder into dense polycrystalline beta alumina, which preferably has a density as close to the theoretical value of 3.26 g./cc. as possible.
There are several drawbacks to methods of this general description. In the first place, they are costly and complicated. For example, two to three milling operations, at least one and often two calcination steps, and up to three drying operations are required to prepare a suitable homogeneous powder. The organic pressing aids that can cause undesirably large pores after burnout and they also add to processing cost. Further, extremely high pressures (.gtoreq.40 Kpsi) are required, as mentioned above. The best automatic commercial presses, however, are designed to operate at only.gtoreq.30 Kpsi and therefore such presses cannot be used to produce beta alumina membranes. Those knowledgeable in ceramic technology will readily appreciate the fact that the cost of pressing would be reduced significantly if lower pressures, for example 10 to 20 Kpsi, could be used. However, it has not been possible to employ such low pressures in the practice of known technology in spite of the fact that improvements in the manufacture of beta alumina electrolytes have been sought by a considerable number of dedicated researchers having access to sophisticated processing and testing facilities.
Spray drying has been introduced to the ceramics industry to replace older techniques for preparing granules which are subsequently pressed into bodies and fired. Typically organic binders are incorporated with a ceramic slip prior to a spray drying operation which replaces a multiplicity of steps required in the practice of more conventional ceramic processing technology. It has been proposed to utilize spray drying as a step in the production of beta alumina ceramic articles. One proposed technique for producing sintered polycrystalline beta alumina articles involves spray drying alpha alumina, an anhydrous relatively inexpensive source of alumina. This processing, designated the "slurry solution" process, is described by Weiner in "RESEARCH ON ELECTRODES AND ELECTROLYTE FOR THE FORD SODIUM-SULFUR BATTERY," National Science Foundation, NSF C805, semi-annual report for 6/30/74-12/31/74, January 1975. As described in this publication, alpha alumina was slurried in concentrated solutions of lithium and sodium nitrates. The slurries were spray dried, producing powders for subsequent calcination. Calcined powder was pressed isostatically at 55 Kpsi and sintered at 1620.degree. C. for 30 minutes, yielding products of 96.7% density. Difficulties were experienced in producing stable slurries amenable to spray drying. Insofar as described, the slurries contained about 37% total solids on a weight basis or about 31% alumina solids. Various suspending agents were used in unsuccessful attempts to overcome the instability of the spray dryer feed slurries but the need for further study was suggested by the author.
In a subsequent development, described in U.S. Pat. No. 4,052,538 to Eddy et al, colloidal alpha-alumina monohydrate was utilized as the source of alumina in a process for producing beta alumina ceramics which featured a spray drying operation. In accordance with the teachings of Eddy et al, a sintered sodium beta alumina article was made by forming an aqueous acidic colloidal solution (sol) of dispersible alpha-alumina monohydrate. An aqueous solution of an oxygen-containing sodium salt and, optionally, a salt of lithium or magnesium, was added to the colloidal solution to form a thixotropic gel which was spray dried to form a free-flowing powder. The powder was pressed, heated to remove volatiles and sintered to form the desired densified beta alumina artlcle. In accordance with one embodiment of the invention, the spray dried powder was calcined and the calcined powder was pressed and sintered. In a preferred embodiment, the powder was pressed before the calcination treatment. As described in the patent, the uncalcined spray dried powder was pressed at.gtoreq.25 Kpsi and the pressed body was slowly heated at about 100.degree. C. per hour to decompose the raw materials and drive off the volatile matter. The article was then sintered at elevated temperature into dense polycrystalline beta alumina. This preferred embodiment represents somewhat of a departure from the prior art since the article was pressed before the precursor powder was calcined and converted to beta alumina. Analysis of the technology, including the preferred embodiment, suggests that it is wasteful of energy because considerable water must be evaporated from the spray dryer feed to produce the desired precursor powder. For example, in the case of the 15% solids slurry of alpha alumina monohydrate described in the illustrative examples of the patent, 85 tons of water would have to be evaporated from 100 tons of spray dryer feed slurry. After volatiles are driven off this would yield only 11 tons of beta alumina. Furthermore, from information provided in the patent to Eddy et al, it can be calculated that the spray dried beta alumina precursor powders would have a relatively low bulk density; i.e., a bulk density that is only 23% of the theoretical for beta alumina. A high compaction ratio would be required to press such a low bulk density powder into a structure having desired high density and this could entrap air in the structure during pressing. Entrapped air can crack the article upon decompression. Beta alumina precursor powders produced by the preferred embodiment of the process will press to low green density values, calculated to be less than 40% of theoretical. Those skilled in ceramics technology will appreciate the fact that low green density results in high shrinkage and, usually, introduces a great potential for cracking and poor dimensional control during firing. Another drawback or limitation to a process such as that of Eddy et al in which the precursor contains hydrated alumina is that a large amount of volatile matter is evolved during heating. This requires that the initial heating rate by low (100.degree. C. per hour until the bodies reach 800.degree. C.) Consequently the production rate would be less than rates achievable if large amounts of volatiles did not need to be eliminated during firing.
A general object of the invention is to provide an economical method of producing a high bulk density beta alumina precursor which possesses superior unfired properties including exceptional green strength and which yields high density polycrystalline beta alumina, preferably beta" alumina, membranes after firing.